Vettel was about 0.4secs quicker than Webber in both the first and third sectors, while the Australian was 0.8secs quicker than the German in the middle sector. That combined to put Vettel just 0.059secs ahead.

Red Bull have traditionally run their car with a higher downforce set-up, optimised for ultimate lap time, but this philosophy has been based on their ability to qualify consistently at the front.

The qualifying pace of the Mercedes cars, which have been on pole for six of the last seven races, and the long straights at Spa which facilitate overtaking have forced them to reconsider that approach.

Ferrari also took a split approach, converting Alonso's car away from the low-downforce set-up used by Massa after the first runs of the session.

He then moved up to seventh from out of the top 10, but did not do a race-simulation run late in the session.

Red Bull, meanwhile, looked as impressive over a series of laps on high fuel levels as they did on their one-off laps.